Why have humans started to be gay Is it very bad if two boys do this When they then become doctor or officer or Engineer Is it very bad to be a gay doctor?

There have been gay people since the beginning of humanity. There is absolutely nothing "Bad' about being gay. It happens to be part of the way we are born, we are either heterosexual, bisexual or homosexual. Some people may not agree with that, it just happens to be a fact.

Whether doctors are gay or not has nothing to do with their ability to heal people, the same goes for any other profession.